Take the Coin Cell Challenge This Weekend!

The year is drawing to a close, and we have a weekend project for you to while away the remaining hours. Take the Coin Cell Challenge!

The point of the challenge is to do something interesting with a coin cell. That’s it! It’s a challenge that can be as simple or as involved as you want. Low power is where it’s at these days, so if you’ve never used the hardware sleep modes in your favorite microcontroller, that would make an excellent challenge entry. Show us what you’re able to do with short wake periods, and talk about when and why that wake happens. Or go a completely different route and build your own cell!

[Ben Krasnow] makes the most of a tiny power sourceThe top twenty entries will each receive a $100 Tindie credit so they can score some excellent gear. Three top winners in some special areas who will each be awarded a $500 cash prize. We’re looking for something interesting that demonstrates longest life (Lifetime Award), something that burns through that coin cell as if it’s going out of style (Supernova Award), and something that fills us with disbelief (Heavy Lifting Award) because it shouldn’t be possible with “just a coin cell”.

7 thoughts on “Take the Coin Cell Challenge This Weekend!”

The energy density of a humble 2032 cell is pretty good. Not quite rocket fuel grade, but it’s up there.

So, silly thought experiment: Can you make a rocket powered by coin cells? Suck the energy from a cell and use that to accelerate the depleted cell out the back. What is the performance relative to actual rockets?

From the spec sheet, a 2032 cell can yield 210 mAh at 2-3 V, or 420-630 mWh. In more convenient units, call it near enough 2 kJ. Expressed in kinetic energy of the 3.2 gram cell being thrown out the aft end, that’s 1118 m/s exhaust velocity.

Dividing by g (9.8 m/s/s) to get the usual (if irrational) units for specific impulse Isp, and saying we can do it with a 90% efficiency (hey, if Tesla can do it, so can I), then we get a coin-cell powered rocket with an Isp of 108 seconds.

Yeah, coin cells have relatively poor energy densities, at least when compared with chemical fuels. According to Wikipedia (not the best source, but the values should be good enough), a kg of lithium batteries (coin cells) constitutes 1.8 MJ, compared to 42.8 MJ from a kg of kerosene, a popular fuel for the early days of rockets. Taking into account a stochiometric ratio of about 4 kg oxygen for every 1 kg of kerosene, and the practical energy density of a kerosene and oxygen is about 8.6. This is still about 4.75 times the energy density of a pile of coin cells. So yeah, no coin cell rockets any time soon.

On a happier note, coin cells still exceed the energy density of Lithium Ion and Polymer batteries that are clawing their way up to 1 MJ/kg energy densities. At least they aren’t supercapacitors, which are apparently beat out by a spinning wheel in terms of energy density.